The present invention relates to a scanning microscope in which an energy beam scans a sample. It is applicable, for example, to a scanning electron microscope, in which the energy beam is made up of electrons, but is also applicable to a scanning microscope using other charged particles, such as ions, and also to a scanning microscope using a laser beam.
In a scanning electron microscope, a sample is scanned by an electron beam and this generates a secondary signal from the sample which is detected and converted to sample image signals. The sample image signals are processed to modulate the luminance of a display image. The display image may be produced, for example, on a cathode ray tube. Scanning occurs by causing the beam to sweep along a series of parallel lines (scanning lines) which define an area on the sample, that area being known as a scanning frame. The distance between the scanning lines is known as the scan interval.
It is usual for the size of the display image to be fixed. Therefore, if the dimensions of the scanning frame of the energy beam on the sample are reduced, the effective magnification of the display image will be increased. Normally, the number of scanning lines in a scanning frame is determined such that the scanning lines of the display are not noticeable when viewed directly by an observer. Thus, they depend on the resolution of the cathode ray tube. Generally, 500-1000 scanning lines are used.
In such an arrangement, the number of scanning lines of the energy beam on the sample within a scanning frame is constant. Therefore, in order to increase the effective magnification, the length of the scanning lines is reduced and the scan interval between the scanning lines is varied.
It is possible to generate a display image directly from the sample image signals. However, it is known to convert the sample image signals into digital signals, and store those digital signals in an image memory. Then, by reading out the content of the image memory, a display image may be generated. With such a technique, it is usual for the image memory to store the sample image signals of only one frame, at any time, and therefore the image memory may also be referred to as a frame memory.
Although it is normal for the energy beam to be focused into a fine spot, in a scanning microscope, the scanning operation described above, in which scanning occurs in a series of parallel scanning lines defining a scanning frame, causes the spot to sweep out a series of areas corresponding to the scanning lines. As the desired magnification is increased, and the scan interval is decreased, the areas swept out for each scanning line approach each other. At a predetermined magnification, the area swept out by the spot along one scanning line will adjoin the area swept out by the spot for the immediately adjacent scanning line. For smaller scan intervals, the areas will overlap. When such overlapping occurs, no further increase in the resolution of the scanned image is possible.
When such overlap occurs, in e.g. an electron microscope, the current density on the specimen may become excessively high. This may cause heating of the specimen and, if the sample is or contains insulating material, may cause accumulation of charged particles on the sample. Furthermore, the build-up of charged particles may affect the sample itself, causing contamination. This problem is particularly acute in the investigation of semiconductor elements using electron microscopes, since charge build-up occurs on insulating layers (such as photoresist), and such charge build-up may affect the properties of the semiconductor element itself. Indeed, such problems may arise even at magnifications below that which corresponds to overlapping of the areas swept out by the spot of the energy beam for adjacent scanning lines.
It may further be noted that problems due to increased current density occur not only during observation of a sample, but also during focusing or astigmatic compensation. Focusing can be achieved with high precision if high magnifications are used. Therefore, even if the sample is to be observed only at relatively low magnifications, focusing may make use of high magnifications, involving magnification of part of the sample. Since the scan interval is thus reduced for the automatic focusing step, the current density will be high for the part of the specimen scanned during the focusing operation. This may damage the specimen or contaminate it. Similar considerations apply to astigmatism correction.